Forward and reverse calibration for ground-based beamforming

ABSTRACT

Methods and systems for calibrating the return and forward links of a satellite communication system are provided according to embodiments of the invention. The phase and/or amplitude variations caused by the return and forward links are calculated and/or estimated to aid in beamforming, such as ground-based beamforming. Calibration earth stations, distributed within one or more beam patterns, may be used to transmit calibration codes to the gateway to calibrate the return link. Return links variations may be estimated using a weighted minimum mean square algorithm at the gateway. Forward links may be calibrated with calibration codes transmitted from the gateway through a hybrid matrix to at least one calibration station. Forward calibration links may also calibrate for temperature-dependent signal variations such as diplexer variations at the satellite.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/682,938 filed Apr. 9, 2015, entitled “Forward and Reverse CalibrationFor Ground-Based Beamforming”, which is a continuation of, and claimspriority to U.S. patent application Ser. No. 13/710,063 filed Dec. 10,2012, issuing as U.S. Pat. No. 9,025,591 on May 5, 2015, which is adivision of, and claims priority to, U.S. patent application Ser. No.12/596,609, filed Mar. 30, 2010, issuing as U.S. Pat. No. 8,331,329 onDec. 11, 2012, which is a U.S. National Phase Application ofPCT/US2007/080720, filed Oct. 8, 2007, which claims the benefit ofcommonly assigned U.S. Provisional Patent Application No. 60/828,539,filed Oct. 6, 2006, entitled “Forward Calibration With Hybrid Matrix ForGround-Based Beamforming,” the entireties of each of which areincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Some satellite systems include mobile satellite programs where multiplespot beams spread over a large geographic area that are engaged toconnect a large number of mobile users to a gateway. Multiple spot beamscan be generated using either a phased array or a reflector antenna withan array feed. Some systems may utilize many feed elements and maybeamform combinations of these to generate multiple beams.

Beamforming is implemented by adjusting the amplitude and phase of eachsignal path routed to each feed element. Each individual signal path isrouted to multiple feeds with relative amplitudes and phases whichdefine each intended beam. On many satellite programs, beamforming isaccomplished by constructing a fixed beamforming network behind the feedarray. Satellite systems have employed an onboard digital signalprocessor (DSP) which performs digital beamforming allowing an entirebeam pattern to be re-optimized at any time during the life of thespacecraft. The DSP, however, adds significant weight and power demandsto the payload. Ground-based beamforming (GBBF) provides the same orgreater flexibility than digital beamforming onboard the satellitewithout the weight and power penalty of an onboard DSP.

GBBF may require knowledge of the phase and/or amplitude variationscaused by the return and forward links. Various satellite components,such as amplifiers, DSP, hybrid matrices, and diplexers, to name a few,may affect the phase and/or amplitude of a signal. There is a need inthe art for ground-based systems and methods that determine the phaseand/or amplitude effects of the forward and return link paths.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to wireless communications in general and,in particular, to forward and return link calibration of satellitecommunication systems that employ GBBF.

A method for estimating the effects of a satellite return link at agateway is provided according to one embodiment of the invention. Themethod includes receiving at a gateway a signal comprising a pluralityof PN codes from a plurality of calibration stations through thesatellite. The satellite receives the PN codes from the calibrationstations through one or more satellite feeds, the satellite retransmitsthe PN codes to the gateway, and at least two of the plurality ofcalibration stations are located within distinct satellite beam coverageareas. The gateway extracts phase information from the received PN codesand then estimates the phase effects of the satellite return link byapplying a weighted minimum mean square algorithm to the extracted phaseinformation.

The weighted minimum mean square algorithm used in the method describedabove uses at least one matrix of nominal feed excitation levels toestimate the phase effects. The method may also include receiving anindication of the satellite orbital position from the satellite,retrieving a matrix of nominal feed excitation levels from memoryassociated with the satellite orbital position and using the retrievedmatrix of nominal feed excitation levels to estimate the phase effects.The method may further include calculating beamforming parameters usingthe estimated phase effects and transmitting the beamforming parametersto the satellite. The method may also extract amplitude information fromthe received PN codes and estimate the amplitude effects of thesatellite return link by applying a weighted minimum mean squarealgorithm to the extracted amplitude information.

Another method for estimating the effects of a satellite return link ata gateway is provided according to another embodiment of the invention.The method may include receiving a signal comprising a plurality of PNcodes from a plurality of calibration stations through the satellite.The satellite receives the PN codes from the calibration stationsthrough one or more satellite feeds, the satellite retransmits the PNcodes to the gateway, and at least two of the plurality of calibrationstations are located within distinct satellite beam coverage areas. Theamplitude information may then be extracted from the received PN codesand the amplitude effects of the satellite return link may be estimatedby applying a weighted minimum mean square algorithm to the extractedamplitude information.

The weighted minimum mean square algorithm used in the method describedabove uses at least one matrix of nominal feed excitation levels toestimate the amplitude effects. The method may also include receiving anindication of the satellite orbital position from the satellite,retrieving a matrix of nominal feed excitation levels from memoryassociated with the satellite orbital position and using the retrievedmatrix of nominal feed excitation levels to estimate the amplitudeeffects. The method may further include calculating beamformingparameters using the estimated amplitude effects and transmitting thebeamforming parameters to the satellite.

Another method for estimating the effects of a satellite return link ona signal at a gateway is provided. The method may include receiving asignal comprising a plurality of PN codes from a plurality ofcalibration stations through the satellite. The satellite receives thePN codes from the calibration stations through one or more satellitefeeds and retransmits the PN codes to the gateway. At least two of theplurality of calibration stations are located within distinct satellitebeam coverage areas. Phase and amplitude information may then beextracted from the received PN codes. The amplitude and phase effects ofthe satellite return link may then be estimated by applying a weightedminimum mean square algorithm to the extracted information.

A satellite communication system is also provided according to oneembodiment of the invention. The satellite communication system mayinclude a plurality of calibration stations, a satellite comprising morethan one feed, and a gateway configured to receive signals from thesatellite. The calibration stations transmit PN signals to the satelliteand the PN signals are received at at least one of the satellite feeds.The gateway receives the PN codes from the satellite. The gateway isconfigured to extract phase information from the PN codes, extractamplitude information from the PN codes, estimate the phase effects of asatellite return link using a minimum mean square algorithm, andestimate the phase effects of a satellite return link using a minimummean square algorithm. The gateway may also include memory that stores aplurality of matrices of nominal feed excitation levels. The gateway mayfurther be configured to receive an indication of the satellite orbitalposition from the satellite, retrieve a matrix of nominal feedexcitation levels from memory associated with the satellite orbitalposition, and use the retrieved matrix of nominal feed excitation levelsto estimate the amplitude effects. The gateway may also be configured tocalculate beamforming parameters using the estimated amplitude effects,and transmit the beamforming parameters to the satellite.

A method for ground-based calibration of a satellite forward link isprovided according to another embodiment of the invention. The methodmay include receiving satellite temperature data at a gateway from thesatellite and determining a phase shift based on the temperature datafrom the satellite. A signal from the gateway may be adjusted based onthe phase shift. The signal may be transmitted to the satellite.

A method for ground-based calibration of a satellite forward link isalso provided according to one embodiment of the invention. The methodmay include receiving satellite temperature data at a gateway from thesatellite and determining an amplitude shift based on the temperaturedata from the satellite. A signal may then be adjusted based on theamplitude shift and transmitted to the satellite.

A method for ground-based calibration of a satellite forward link isprovided according to another embodiment of the invention. The methodmay include transmitting a plurality of orthogonal codes from a gatewayto a satellite that are received at a unique feed at the satellite. Theplurality of orthogonal codes are multiplexed into a linear combinationof orthogonal codes at the satellite and transmitted to a plurality ofcalibration earth stations over a plurality of feeds. At least one ofthe calibration earth stations receives the linear combination oforthogonal codes with significant signal to noise ratio where theorthogonal codes are demultiplexed and the phase and/or amplitudevariations are determined.

Another method for ground-based calibration of a satellite forward linkis disclosed according to one embodiment of the invention. The methodmay include transmitting at least one PN code from a gateway to asatellite where the PN code is transmitted from the satellite through aplurality of feeds to at least one calibration earth station. The PNcode is transmitted through each feed consecutively one after another.The PN codes are transmitted for 10-100 ms up to 1-10 seconds over eachfeed.

Yet another method for ground-based calibration of a satellite forwardfeeder link is provided according to another embodiment of theinvention. The method may include transmitting at least one coded signalto a calibration earth station through a satellite system. The phaseshift and/or amplitude shift is determined from the received codedsignal at the calibration earth station. Satellite temperature data isalso received at the calibration earth station from the satellite. Thetemperature dependent phase shift and/or temperature dependent amplitudeshift is determined based on the temperature data from the satellite.The temperature dependent phase shift and/or the temperature-dependentamplitude shift is subtracted from the phase shift and/or amplitudeshift, thereby providing the forward feeder link phase shift and/oramplitude shift.

Another method for ground-based calibration of a satellite forward linkis provided according to another embodiment of the invention. The methodmay include transmitting a signal including at least one predeterminedcode from a gateway to at least one calibration earth station through asatellite. The signal may include a plurality of orthogonal calibrationcodes and is transmitted from the satellite to the calibration earthstation through at least one feed. The signal is then received atcalibration earth station and compared with a predetermined code. Thephase and/or amplitude shift may then be determined. Beamformingparameters may be calculated using the phase and/or amplitude shifts andthen transmitted to the satellite. The phase and/or amplitude shift maybe determined at the gateway or a calibration earth station.

A satellite is provided according to another embodiment of theinvention. The satellite may include a receiver, one or more amplifiers,an output hybrid matrix, one or more diplexers and a transmitter. Thereceiver is configured to receive signals from one or more gateways andmay include one or more feeds. The one or more amplifiers are coupledwith the one or more receiver feeds and the output hybrid matrix. Theoutput hybrid matrix may include one or more output ports that arecoupled with the diplexers. A temperature sensor may be included thatmeasures the temperature at or near the one or more diplexers. Thetransmitter is configured to transmit signals received from the one ormore gateways to one or more receivers and to transmit temperature datafrom the temperature sensor to the one or more gateways. The temperaturesensor may include one or more temperature sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a satellite system configuration according to oneembodiment of the invention.

FIG. 2 shows a functional block diagram of a calibration earth stationaccording to one embodiment of the invention.

FIGS. 3A-3E are three a block diagrams showing portions of a return linkcalibration according to one embodiment of the invention.

FIG. 4A-4E are three a block diagrams showing portions of a forward linkcalibration according to one embodiment of the invention.

FIG. 5 shows a block diagram of two calibration earth stationscommunicating with a gateway through two feeds on a satellite accordingto one embodiment of the invention.

FIG. 6 shows a flowchart of a method for determining beamformingparameters based on the phase and amplitude effects of a satellitereturn link according to one embodiment of the invention.

FIG. 7A shows a hybrid matrix used in a ground-based beamforming (GBBF)satellite system according to one embodiment of the invention.

FIG. 7B shows a hybrid matrix with calibration codes input after theinput hybrid matrix (IHM) as used in a ground-based beamforming (GBBF)satellite system according to one embodiment of the invention.

FIG. 7C shows a hybrid matrix with calibration codes input at the inputports of the IHM as used in a GBBF satellite system according to oneembodiment of the invention.

FIG. 8 shows a series of spot beam patterns from a satellite with acalibration earth station (CES) within each spot beam.

FIG. 9 shows a series of spot beam patterns from a satellite with a CESused for a plurality of spot beam patterns according to one embodimentof the invention.

FIG. 10 shows an output hybrid matrix (OHM) with diplexers and atemperature sensor according to one embodiment of the invention.

FIG. 11 shows a flowchart depicting a method for calibrating the forwardlink of a satellite system using temperature information from thesatellite according to one embodiment of the invention.

FIG. 12 shows a flowchart depicting a method for calibrating the forwardlink of a satellite system using orthogonal calibration codes accordingto one embodiment of the invention.

FIG. 13 shows a flowchart depicting a method for calibrating the forwardservice link of a satellite system according to one embodiment of theinvention.

FIG. 14 shows a flowchart depicting a method for calibrating the forwardlink of a satellite system using orthogonal calibration codes and usingtemperature information from the satellite according to one embodimentof the invention.

FIG. 15 shows a flowchart depicting a method for calibrating the forwardlink of a satellite system and using the calibration information toproduce beamforming parameters according to one embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention comprise systems, methods,devices, and software for a novel broadband satellite network. Thisdescription provides exemplary embodiments only, and is not intended tolimit the scope, applicability or configuration of the invention.Rather, the ensuing description of the embodiments will provide thoseskilled in the art with an enabling description for implementingembodiments of the invention. Various changes may be made in thefunction and arrangement of elements without departing from the spiritand scope of the invention.

Thus, various embodiments may omit, substitute, or add variousprocedures or components as appropriate. For instance, it should beappreciated that in alternative embodiments, the methods may beperformed in an order different than that described, and that varioussteps may be added, omitted or combined. Also, features described withrespect to certain embodiments may be combined in various otherembodiments. Different aspects and elements of the embodiments may becombined in a similar manner. Also, a number of steps may be requiredbefore, after, or concurrently with the following embodiments.

FIG. 1 shows a satellite system configuration 100 according to oneembodiment of the invention. A satellite 105 provides a communicationlink between two gateways 110 and three calibration earth stations (CES)115. While only a single satellite 105 is shown, a plurality ofsatellites may be used. Similarly, more than two gateways may beemployed and any number of CESs may also be employed according toembodiments of the invention. For example, four gateways, may be used tocover the continental United States. Other receivers and transmitters(not shown) may also be in communication with the gateways 110 throughthe satellite 105. Each CES 115 is coupled to a communication network120, for example, the Internet, which may also be coupled with thegateways 110.

Each gateway 110 may use a satellite antenna to bi-directionallycommunicate with a satellite 105 on a feeder link 120. A feeder link 120communicates information from the gateway 110 to the satellite 105, andanother feeder link 125 communicates information from the satellite 105to the gateway 110. The satellite 105 may be a bent-pipe. In otherembodiments the satellite may perform switching. The feeder links 120,125 may operate, for example, in the Ku-band. The service links 130, 135may operate, for example, in the L-band or the S-Band. Signals receivedat the satellite 105 from the CESs 115 in the L or S-Band may beretransmitted to the gateway(s) 110 in the Ku-Band.

Information may bi-directionally pass through the satellite 105 from thegateways, CESs or other transmitters/receivers. The satellite 105 coulduse antennas or phased arrays when communicating. The satellite 105 mayalso include multiple antenna elements. One or more antenna elements mayreceive a signal from a CES 115. These multiple antenna elements mayprovide multiple feeds from which signals may be received.

The CESs 115 may provide support for a ground-based beam-forming (GBBF)subsystem at the gateways 110. The CESs 115 may transmit and/or receivesignals to or from the gateways 110 that may be used to provide bothforward and reverse beam-forming at the satellite 105.

CESs 115 may be placed at each gateway site and may be strategicallyplaced in areas of satellite coverage. For example, a satellite 105 mayprovide coverage for the continental United States, Hawaii and populousareas of Canada and Alaska. Such systems may include four gatewaysspread throughout the geography that communicate with one or twosatellites. These satellites singly or in combination may then cover thereceivers within various coverage areas. The satellites may provide aplurality of spot beams each of which cover a portion of the geography.At least one CES may be positioned within each spot beam coverage areato provide return and forward link information to the gateway forcreating satellite beam-forming parameters. Each CES may have a higherequivalent isotropically radiated power (EIRP) than the typical userterminal. Moreover, the CESs may be positioned throughout the geographythat they have an unobstructed view of the satellite or satellites.

FIG. 2 shows an exemplary CES 115 according to one embodiment of theinvention. According to this embodiment of the invention, the CESemploys two antennas 205 to allow transmit signals with differentpolarization. The CES may also include a high power amplifier (HPA), alow noise amplifier (LNA) and or various filters 210 that condition thesignal prior to transmission at the antennas 205. A baseband unit 215,as known in the art, may also be included. A router 220 and terrestriallink 225 may provide communication access to the network. In oneembodiment of the invention, each CES may transmit a unique code to thegateway that is used at the gateway to calibrate the satellite returnlink. These unique codes may be orthogonal to each other. Moreover, thecodes may be pseudo-noise sequences (PN codes). For example, each codemay be a column or row from a Walsh Matrix.

FIGS. 3A-3E are block diagrams showing various portions of a return linkcalibration scheme according to one embodiment of the invention. Turningfirst to FIG. 3A, which shows various fast effects of the return link.Pilot signals are transmitted through the return link and undergoDoppler and/or oscillator shifts as shown in block 305. Doppler shiftsare caused by the motion of the satellite relative to the gateway and/orthe CESs. Oscillator shifts are shifts related to differences in theoscillators at the CESs, gateway and/or the satellite. The fast erroreffects is determined and returned as E_(f)(t). Those skilled in the artwill recognize how to extract these Doppler and oscillator amplitude andphase shifts.

FIG. 3B shows a block diagram detailing various effects encountered byPN codes transmitted by a CES to a gateway through the return link.These effects are commonly called medium effects. These effects mayinclude medium Doppler effects 310, oscillator and propagation effects315, and various medium effects from hardware, antenna, etc 320.

FIG. 3C shows a block diagram of a method used to track the mediumeffects and produce beamforming coefficients (BFC) according to oneembodiment of the invention. PN codes from a CES are received at thegateway. The fast error effects, E_(f)(t), are first removed at block325. The resulting signal is then beamformed at block 330. Thebeamforming increases the strength of the signal from which the mediumeffects, E_(m)(t), may be extracted at block 335. Those skilled in theart will recognize various ways to extract medium effects from the PNcodes. The resulting signal, after removal of the fest effects,E_(f)(t), is also sent to block 340 where the medium effects, E_(m)(t)are removed. The PN codes are correlated at block 345 and run throughaveraging loops at block 350 after which BFC are returned.

FIG. 3D shows a block diagram detailing various effects encountered bydata traffic transmitted by a CES to a gateway through the return link.The data traffic encounters the same channel effects as the PN codes.FIG. 3E shows a block diagram for removing the estimated path effectsfrom a signal and applying BFC. The fast effects are removed at block355. Following which the medium effects are removed at block 360.Ground-based beam forming may then occur at block 365 by applying theBFCs. Accordingly, signals from the various transmitters may transmitsignals including data traffic to the gateway through a satellite. Thesesignals are beamformed at the gateway, after transmission using a methodsuch as the one depicted in FIG. 3E.

FIG. 4A-4E are block diagrams showing portions of a forward linkcalibration according to one embodiment of the invention. Turning firstto FIG. 4A, pilot signals are transmitted through the forward link fromthe gateway to one or more CES and undergo Doppler and/or oscillatorshift as shown in block 405. The fast effects are determined at the CESand returned as E_(f)(t). Those skilled in the art will recognize how toextract these Doppler and oscillator amplitude and phase shifts.

FIG. 4B shows a block diagram detailing various effects encountered byPN codes transmitted by a gateway to a CES through a forward link. Theseeffects are commonly called medium effects. These effects may includeDoppler effects 410, oscillator and propagation effects 415, and variousslow effects from hardware, antenna, etc 420.

FIG. 4C shows a block diagram of a method used to track the mediumeffects and produce BFCs according to one embodiment of the invention.PN codes transmitted from a gateway are received at a CES. The fasterror effects, E_(f)(t), are first removed at block 425. The resultingsignal is then beamformed at block 430. The beamforming increases thestrength of the signal from which the medium effects, E_(m)(t), may beextracted at block 435. Those skilled in the art will recognize variousways to extract medium effects from the PN codes. The resulting signal,after removal of the fest effects, E_(f)(t), is also sent to block 440where the medium effects, E_(m)(t) are removed. The PN codes arecorrelated at block 445 and run through averaging loops at block 450after which BFCs are returned.

FIG. 4D shows a block diagram for removing the estimated path effectsfrom a signal and applying the BFC. On the forward channel thecorrections are made prior to transmission of the signal. Accordingly,error correction due to the path occurs prior to encountering theerrors. The fast effects are removed from a signal at block 455.Following which the medium effects are removed at block 460. Forwardlink ground-based beam forming may then occur at block 465 by applyingthe BFCs. The traffic may then be transmitted to the CES through thesatellite.

FIG. 4E shows a block diagram detailing various effects encountered bydata traffic transmitted by a gateway to a CES through the forward link.These are the same effects encountered by the PN codes and shown in FIG.4B. Precorrection has already occurred on the signal for these effects.

Return Link Calibration

FIG. 5 shows a block diagram of two calibration earth stations 115-i,115-j communicating with a gateway 110 through two feeds on a satellite105 according to one embodiment of the invention. As shown, thesatellite 105 receives two signals at two separate antenna elements fromeach of the two CESs. For instance, CES_(j) 115-j transmits signal θ_(j)that is received at feed n as signal θ_(jn). The signal is also receivedat feed m as signal θ_(jm). Similarly CES_(i) transmits a signal θ_(i)that is received at feed n as signal θ_(in). The signal is also receivedat feed m as signal θ_(im). These signals, θ_(i) and θ_(j) may beorthogonal PN codes. The satellite may be equipped with a plurality offeeds. Each signal may be received at any number of feeds. In somecases, a signal from a CES may be received at a single feed, at twofeeds or at three feeds up to being received at every feed. Accordingly,feed_(n) and feed_(m) comprise signals from both CESs 115. These feedsare then transmitted to the gateway 110 where they may be used tocalibrate the satellite return link and/or for generating beamformingparameters for the satellite.

FIG. 6 shows a flowchart for generating beamforming parameters from thereceived PN codes according to one embodiment of the invention. PN codesare received from the satellite at block 605. The PN codes may comprisePN codes from a plurality of CESs transmitted through a plurality offeeds at the satellite. Each of the antenna elements on the satellitereceives significant excitation from at least one CES. In an exemplarysystem, roughly half the elements receive significant excitation from 2or more CESs. Table 1 shows an exemplary table of signals received atthe gateway from four CESs through 10 satellite feeds. As shown, not allfeeds are illuminated by all of the CESs. By processing the PN codes atthe gateway, individually before beamforming, the function of slowcalibration can be performed to characterize all element paths. Theblank entries in the table indicate that the PN code originated from agiven CES (beam) is not received with sufficient quality by acorresponding feed element, and is therefore discarded at the satelliteor at the gateway. Conversely, the entries in the table indicate signalsof sufficient quality that are included in the optimization thatfollows. When the table is fully populated, each feed can see more thanone CES station

TABLE 1 Exemplary table of PN codes received from 4 CESs through 10satellite feeds. CES₁ CES₂ CES₃ CES₄ Feed₁ PN₁₂ PN₁₃ Feed₂ PN₂₄ Feed₃PN₃₃ Feed₄ PN₄₁ PN₃₄ PN₄₄ Feed₅ PN₅₄ Feed₆ PN₆₁ PN₆₂ Feed₇ PN₇₄ Feed₈PN₈₃ Feed₉ PN₉₄ Feed₁₀ PN₁₀₁ PN₁₀₃

The phase and/or amplitude effects of the satellite return link may beextracted from the PN codes at block 610. The phase and/or amplitudeeffects may occur from one or two sources at the satellite. As shown inFIG. 5, the path from a CES 115 to the satellite including the satellitereceiver may provide one source of phase shift and/or amplitude changes.These effects are denoted as θ. The path from the satellite 105 to thegateway 110 including the satellite transmitter may prove a secondsource of phase shift and/or amplitude changes. These effects aredenoted as φ. Accordingly, this phase shift and/or amplitude changesinformation may be extracted from the received PN codes. Thisinformation may be extracted by comparing the transmitted PN codes withthe received PN codes. Table 2 shows the return link phase shiftsassociated with the received PN codes. A similar table may also beconstructed for the return link amplitude changes.

TABLE 2 Exemplary table of phase return link phase shifts. CES₁ CES₂CES₃ CES₄ Feed₁ φ₁ + θ₂ φ₁ + θ₃ Feed₂ φ₂ + θ₄ Feed₃ φ₃ + θ₃ Feed₄ φ₄ +θ₁  φ₄ + θ3 Feed₅ Feed₆ φ₆ + θ₁ φ₆ + θ₂ Feed₇ φ₇ + θ₄ Feed₈ φ₈ + θ₃Feed₉ φ₉ + θ₄ Feed₁₀ φ₁₀ + θ₁  φ₁₀ + θ₃ 

Embodiments of the invention provide for methods and systems thatperform estimations of these phase shifts jointly rather than estimatingthese phase shifts one at a time. Returning to FIG. 6, estimation of thephase shift occurs at block 630. The phase shift is a function that canbe based on the excitation level at the antenna elements at thesatellite. Because the gain of the various satellite antenna elements asseen from fixed CES locations change, these excitation levels depend onthe orbital position of the satellite. Accordingly, in one embodiment ofthe invention, the orbital position of the satellite is determined atblock 615. That is the satellite may communicate orbital position to thegateway or the gateway may track the satellite and determine orbitalposition. A matrix of excitation levels based on the orbital positionmay be retrieved from the memory 625 at the gateway at block 620. Thematrices may change over time. Accordingly, the matrices may be updatedperiodically. The satellite may communicate

The phase shift caused by the return link may then be estimated, forexample, by minimizing the optimal weighted minimum mean square (WMMS)error. The weights are based on the feed excitation levels in the matrixof excitation levels. The WMMS estimator generates a system of linearequations in the variables to be estimated. The matrix of the linearsystem of equations resulting from the WMMS depends only on the nominalfeed excitation levels.

Once the phase shifts are estimated, the return link may be calibrated.In one embodiment of the invention, beamforming parameters may becalculated and/or adjusted based on the known return link phase shiftsin block 635. At block 640, these parameters are transmitted to thesatellite. As shown in the flowchart, the amplitude effects may also beestimated using a similar scheme. Any beamforming techniques may be usedto calculate the beamforming parameters using the phase and/or amplitudecorrections.

If the error in estimating entry ij of the 2 dimensional array bedenoted by e_(ij), and if Θ_(i) and Φ_(j) indicate the estimate of θ_(i)and φ_(j), respectively, then

e _(ij)=θ_(i)+φ_(j)−(Θ_(i)+Φ_(j)).   eq. 1

the weighted sum of squares of the errors e_(ij) can be written as

$\begin{matrix}{{{E^{2} = {\sum\limits_{i}^{I}\; {\sum\limits_{j}^{J}\; {w_{ij}e_{ij}^{2}}}}},}\;} & {{eq}.\mspace{14mu} 2}\end{matrix}$

where the w_(ij) are weights chosen proportional to the signal-to-noiseratio corresponding to entry ij of the array. That is, w is the matrixof excitation levels. The blank entries in the array (see Table 1) aregiven a weight of zero. Taking derivatives of E² with respect to θ_(i)and to φ_(j), and then those derivatives may be set to zero, as iscommonly done in minimum mean square estimation.

$\begin{matrix}{{\frac{\partial E^{2}}{\partial\theta_{i}} = {{0\mspace{14mu} {for}\mspace{14mu} i} = 1}},2,\ldots \mspace{14mu},I,} & {{eq}.\mspace{14mu} 3} \\{{\frac{\partial E^{2}}{\partial\varphi_{j}} = {{0\mspace{14mu} {for}\mspace{14mu} j} = 1}},2,\ldots \mspace{14mu},{J.}} & {{eq}.\mspace{14mu} 4}\end{matrix}$

Eq. 3 and eq. 4 return a set of I+J linear equations with I+J unknowns.Now, let

$\begin{matrix}{{A_{i} = {\sum\limits_{j}^{J}\; {w_{ij}\left( {\theta_{i} + \varphi_{j}} \right)}}},{and}} & {{eq}.\mspace{14mu} 5} \\{B_{j} = {\sum\limits_{i}^{I}\; {{w_{ij}\left( {\theta_{i} + \varphi_{j}} \right)}.}}} & {{eq}.\mspace{14mu} 6}\end{matrix}$

Substituting the A_(j) and B_(j) into eq. 3 and eq. 4 a matrix equationcan be returned

P·W=C.   eq. 7

Where W is a matrix whose coefficients are w_(ij) and P and C arevectors of the following form:

P=[Θ₁, Θ₂, . . . , Θ_(I), Φ₁, Φ₂, . . . , Φ_(J)], and   eq. 8

C=[A₁, A₂, . . . , A_(I), B₁, B₂, . . . , B_(J)]  eq. 9

The matrix W is known and C is a vector of known quantities.Accordingly, the estimated phase shifts Φ_(j) and Θ_(i) can then bedetermined using P=W ¹C . The matrix W may be inverted and stored inmemory for fast processing at the gateway.

The preceding discussion of the WMMS dealt with the estimation of phasefrom the received PN codes. Phase variations arising from differentsources along the return link are added to produce the overall phasechange. Amplitude variations arising from different sources aremultiplied to produce the overall amplitude change. When measured indBs, amplitude changes are added to produce the overall amplitudechange. Mathematically amplitudes in dBs can be treated similarly tophases in degrees or radians, and a similar procedure to the WMMSoutlined above can be used for estimating amplitudes in dBs. PN codesmay be continuously transmitted over the forward channel along with dataand other communications.

Forward Link Calibration

FIG. 7A shows a hybrid matrix used in a ground-based beamforming (GBBF)satellite system according to one embodiment of the invention. Thehybrid matrix includes an input hybrid matrix (IHM) 710 at the gateway110, power amplifiers 730 and an output hybrid matrix (OHM) 720 at thesatellite 105. In the case shown in the figure, the IHM 710 receivesfour signals A, B, C and D, at the input of the IHM 710. The individualsignals, A, B, C and D, are combined in a linear transformation anduplinked through a feeder link 120 to the satellite 105. The signals arethen individually amplified at a series of power amplifiers 730 and theamplified signals are input at the input ports of an OHM 720. Thesignals are recombined as a linear combination of signals at the OHM 720and then downlinked through a service link 130 to receivers using theproper antenna and downlinks. The OHM 720 essentially performs theopposite function as the IHM 710. Accordingly, each receiver receives alinear combination of the four signals. While a 4×4 IHM and a 4×4 OHMare shown, any number of ports may be used on the IHM and OHM. IHM andOHM commonly have ports with a factor of 2.

Both the IHM 710 and OHM 720 may include a plurality of 3 dB couplers.The IHM operation may be performed in the digital domain, beforeconversion to analog.

As shown in the figure, the OHM will contain a linear combination of the4 input signals. Each output port has the same signal components, butwith different phase rotations which are multiples of 90°. The i^(th)output of the IHM is labeled ABCD^(i).

Embodiments of the invention provide calibration for the variousportions of the forward link from the gateway 110 through the satellite105 to receivers, such as calibration earth stations (CESs) 115. Variouscalibration schemes, techniques, methods and systems are providedthroughout this disclosure.

In one embodiment of the invention, orthogonal codes are used tocalibrate all or some of the forward link. FIG. 7B shows a hybrid matrixwith pseudo-noise (PN) codes input after the IHM at the gateway. Each ofthe calibration codes is orthogonal to each other PN code. That is, thedot product of PN^(i) and PN^(j) equals 0, where i≠j. While calibrationcodes are used in this example, any type of orthogonal coding scheme maybe used. For example, the calibration codes may be derived from columnsof a Walsh matrix. The calibration codes may include a series of valuesthat include either +1 or −1. Unique calibration codes are introducedafter the outputs of the IHM 710 and are transmitted to the satellite105 through the forward uplink 120. As will be discussed further, thecalibration codes may be used by the satellite system to calibrate theforward link of the system. The calibration information may then be usedto provide ground-based beamforming parameters.

The calibration codes may be transmitted at the same time as other datais being transmitted to the satellite and then to various receivers.Because the codes are pseudo-noise codes, the calibration codes will notinterfere with the transmitted data. Moreover, calibration codes mayalso be transmitted from the gateway to the satellite at times when datais not being transmitted.

FIG. 7C shows a hybrid matrix with calibration codes input at the inputports of the IHM as used in a ground-based beamforming (GBBF) satellitesystem according to one embodiment of the invention. As shown, fourcalibration codes PN¹²³⁴ are input at each port of the IHM 710 and thefour calibration codes are returned from the IHM 710 and uplinked to thesatellite 105.

As shown in FIGS. 7B and 7C, each PN code is output from the OHM as alinear combination of calibration codes. Thus, each CES receives each PNcode from the satellite. Thus, a single CES that receives the linearcombination of calibration codes may determine the phase and/oramplitude effects of the forward link.

If the satellite does not output each PN code as a linear combination ofthe calibration codes on each feed then a CES is required in each andevery spot beam as shown in FIG. 8. However, employing embodiments ofthe present invention each feed provides a linear combination ofcalibration codes, then a single CES may be employed that receives eachof the calibration codes as shown in FIG. 9. Four spot beam patterns 910are shown. Each spot beam receives the linear combination of signalsshown as ABCD in the figures. Spot beam 1 910-A receives calibrationcodes ABCD¹. Spot beam 2 910-B receives calibration codes ABCD². Spotbeam 3 910-C receives calibration codes ABCD³. Spot beam 4 910-Dreceives calibration codes ABCD⁴. In this example, only spot beam 1includes a CES 115. Because each feed from the satellite includes everycode, only one CES is required to calibrate the forward link. Of course,for redundancy, quality and interference purposes, more than one CES maybe used over a number of spot beams.

The CES may be required, in one embodiment of the invention, to receivethe calibration codes ABCD with sufficiently high signal-to-noise ratio.Moreover, the power assigned to each code is selected such that allcodes are received at approximately the same signal level at the CES.That is, weak feeds are assigned more power than strong feeds. Sincethese codes are all different, for example without beamformingadvantage, and since they lose 6 dB in the hybrids (they appear on all 4ports of the hybrids, but only one port is useful in general), it may bebeneficial to boost the power assigned to those codes. The totalsatellite power allocated to these forward-calibration PN codes isgenerally significantly less than 1% of the total satellite power. Inother embodiments of the invention the satellite power is less than0.5%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4% 4.5% or 5%.

In addition to the orthogonal calibration codes used for calibrating theforward links and/or paths, reference signals may be sent to bebeamformed in the far field at the CES. The reference signals may betransmitted with more power than comparable traffic signals of similartransmission rate. The purpose of the reference signals is to track thestatus of the channel for the medium calibration stage. This allows forcode correlation and post-correlation averaging of relatively longdurations (on the order of 100 milliseconds) for each of the calibrationcodes that perform the function of slow calibration.

At the CES 115, the phase and/or amplitude of the received PN code(s)may be compared with the phase and/or amplitude of the calibration codessent by the gateway. The CES may receive the originally transmittedcalibration codes or information about the phase and/or amplitude of thecalibration codes from the gateway through a terrestrial link (as shownin FIG. 1) or through the satellite. The calibration codes may also bedetermined prior to implementation in the system. Accordingly, the phaseand amplitude shift in each of the calibration codes may be used tocalibrate aspects of the forward link. The CES may transmit the phaseand/or amplitude shift data back to the gateway through the satellite orthrough a terrestrial link. In another embodiment of the invention, thecalibration codes may be transmitted to the gateway, which may be usedat the gateway to determine beamforming parameters. For example,beamforming parameters for an antenna array at the satellite may becalculated at the gateway. The proper phase and amplitude adjustments tothe signal may, therefore, occur at the gateway. The phase and/oramplitude shift from the forward link as found through the calibrationcodes may be applied to the beamforming calculation.

Placing the IHM at the gateway may require that the paths from thegateway to the OHM and the paths after the OHMs be calibratedseparately. If it were possible to characterize the temperaturedependence of any diplexers placed after the OHMs, then calibrationafter the OHMs may not be needed. Otherwise, calibration of the pathsafter the OHMs is needed, although changes along those paths may betypically slow. Accordingly, embodiments of the invention provide forcalibration of the paths from IHM to OHM, calibration of paths after theOHM without diplexers, and calibration of the paths after the OHM withdiplexers.

FIG. 10 shows an OHM 720 with diplexers 1010 and a temperature sensor1020 according to one embodiment of the invention. The phase and/oramplitude effects on a signal or signals through the diplexers 1010 maydepend on the temperature of the diplexer. As such, these effects may becharacterized prior to deployment of the satellite and the temperatureversus the phase and/or amplitude shift may be stored in a look-up tableat the gateway. More than one temperature sensor may be employed at thesatellite. In one embodiment, a temperature sensor may measure thetemperature of each diplexer. In another embodiment of the invention,one or more temperature sensor may measure the temperature of a group ofdiplexers.

As shown in FIG. 11, the temperature sensor 1020 on the satellite mayrecord the temperature of diplexer or diplexers 1010 and transmit thetemperature to the gateway on a return feeder link at block 1105. Thegateway may receive the temperature at block 1110 and then compensatefor these phase and/or amplitude effects by looking up the phase and/oramplitude shift in the lookup table 1125 using the temperature at block1115. The phase and/or amplitude shift(s) may be applied to beamformingcalculations at a GBBF at block 1120. The temperature of the diplexersmay be measured with a single temperature sensor and may be used tolook-up phase and/or amplitude variation data. In one embodiment of theinvention, the temperature-related variations are determined for a groupof diplexers using the look up table. In another embodiment, eachdiplexer uniquely affects the phase and/or amplitude and each diplexerhas a unique lookup table. The phase and/or amplitude variations maythen be determined one diplexer at a time.

FIG. 12 shows a flowchart depicting a method for calibrating the forwardlink of a satellite system using orthogonal calibration codes accordingto one embodiment of the invention. A plurality of calibration codes,for example PN Codes, are transmitted to the satellite at block 1205.Each PN code is transmitted to a separate feed of the satellite. Thecalibration codes may be added before or after an IHM as shown in FIGS.7B and 7C. The calibration codes may be transmitted to the satelliteover separate paths and/or to distinct satellite feeds.

The calibration codes are received at the satellite at block 1210 andmultiplexed through an OHM at block 1215, creating a linear combinationof the calibration codes. This linear combination of calibration codesis then transmitted from each and every satellite feed at block 1220.The satellite may receive the calibration codes at block 1210 in onefrequency band and transmit the codes in the same or a different band atblock 1220.

The linear combination of calibration codes is received at each andevery receiver including the CES within the spot beams of the feeds fromthe satellite at block 1225. Because a linear combination of calibrationcodes is transmitted from each and every feed of the satellite, any oneCES may receive the calibration codes and separate the linearcombination of calibration codes into the separate calibration codes atblock 1230. A CES that receives the calibration codes may then determinethe amplitude and/or phase shifts of the calibration codes at block1235. The CES may compare each calibration code with the knowncalibration codes transmitted from the gateway. The known calibrationcodes may be stored at the CES and/or received at the CES through aterrestrial link. The amplitude and/or phase shifts may be communicatedto the satellite and/or the gateway through the satellite or through aterrestrial link.

FIG. 13 shows a flowchart depicting a method for calibrating the forwardservice link of a satellite system according to one embodiment of theinvention. In this embodiment it is not assumed that the phase and/oramplitude variations after the OHMs are negligible or could be inferred.The phase and/or amplitude variations after the OHMs vary slowly. Thesevariations may be calibrated separately.

In one embodiment of this aspect of the subject invention, a monitoringPN code is assigned for this purpose. The monitoring PN codes aretransmitted to and received by the satellite at blocks 1305 and 1310.The monitoring code may be transmitted from the gateway in one IHM at atime, in such a way that it excites, one after the other, each of thefeeds of the corresponding OHM at the satellite at block 1315 andreceived at the CES at block 1320. By assigning more monitoring time ormore power for the weaker feeds (as seen by the CESs), all feeds can bemonitored from a few CES receiver locations. The phase and/or amplitudeshifts and/or corrections may then be determined at the CES at block1325. This periodic monitoring of variations after the OHMs, when usedin combination with the forward calibration signals, may calibrate forall forward path variations (before and after the OHMs). If thevariations after the OHMs are rapid, then all CESs could be used asreceivers. However, slow variation is generally the case in practice.

FIG. 14 shows a flowchart depicting another method for calibrating theforward link of a satellite system using orthogonal calibration codesand using temperature information from the satellite according to oneembodiment of the invention. Calibration codes are transmitted from agateway to one or more CES through a satellite at block 1405. The phaseand/or amplitude shift from the received coded signal may be determinedat the CES at block 1410. In another embodiment of the invention, theCES sends the received coded signals to the gateway where the phaseand/or amplitude shifts are determined. Satellite temperature data arereceived from the satellite at block 1415 from which the temperaturedependent phase and/or amplitude effects may be determined at block1420. The temperature-dependent phase and/or amplitude shifts may besubtracted from the other measured phase and/or amplitude shifts atblock 1425. Thus, the effect of the forward calibration path on thephase and/or amplitude without the effects of the temperature effectsmay be determined. Because non-temperature dependant effects may beslowly changing, a system may determine these effects less often whiledetermining the temperature effects more often.

FIG. 15 shows a flowchart depicting a method for calibrating the forwardlink of a satellite system and using the calibration information toproduce beamforming parameters according to one embodiment of theinvention.

In many instances the phase and/or amplitude variations after OHMs werenegligible or could be inferred (for instance by temperature monitoringand reporting). Such variations typically vary relatively slowly. In oneembodiment of this aspect of the subject invention, a monitoring PN codeis assigned for this purpose. The monitoring code is transmitted fromthe gateway in one IHM at a time to a satellite. The monitoring code,may be, for example, a PN code. The code is received at the satellite.The monitoring code may be transmitted to the satellite in such a waythat it excites, one after the other, each of the 4 feeds of thecorresponding OHM. By assigning more monitoring time or more power forthe weaker feeds, as seen at the CES, all feeds can be monitored from afew CES locations. The monitoring code may be received at a CES wherethe received code is compared with the transmitted code. The phaseand/or amplitude variations may be determined. Using the phase and/oramplitude variations beamforming parameters may be calculated andtransmitted to the satellite. This periodic monitoring of variationsafter the OHMs, when used in combination with the forward calibrationsignals discussed above, calibrate for all forward path variations(before and after the OHMs). If the variations after the OHMs are rapid,then all CES, may be used as receivers. However, slow variation isgenerally the case in practice.

It should be noted that the systems, methods, and software discussedabove are intended merely to be exemplary in nature. It must be stressedthat various embodiments may omit, substitute, or add various proceduresor components as appropriate. For instance, it should be appreciatedthat in alternative embodiments, the methods may be performed in anorder different than that described, and that various steps may beadded, omitted or combined. Also, features described with respect tocertain embodiments may be combined in various other embodiments.Different aspects and elements of the embodiments may be combined in asimilar manner. Also, it should be emphasized that technology evolvesand, thus, many of the elements are exemplary in nature and should notbe interpreted to limit the scope of the invention.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, it will be understood by oneof ordinary skill in the art that the embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, algorithms, structures, and techniques have been shownwithout unnecessary detail in order to avoid obscuring the embodiments.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flowchart, a structure diagram, or a blockdiagram. Although they may describe the operations as a sequentialprocess, many of the operations can be performed in parallel orconcurrently. In addition, the order of the operations may bere-arranged. A process is terminated when its operations are completed,but could have additional steps not included in the figure.

Moreover, as disclosed herein, the terms “storage medium” or “storagedevice” may represent one or more devices for storing data, includingread only memory (ROM), random access memory (RAM), magnetic RAM, corememory, magnetic disk storage mediums, optical storage mediums, flashmemory devices or other computer readable mediums for storinginformation. The term “computer-readable medium” includes, but is notlimited to, portable or fixed storage devices, optical storage devices,wireless channels, a sim card, other smart cards, and various othermediums capable of storing, containing or carrying instructions or data.

Furthermore, embodiments may be implemented by hardware, software,firmware, middleware, microcode, hardware description languages, or anycombination thereof. When implemented in software, firmware, middlewareor microcode, the program code or code segments to perform the necessarytasks may be stored in a machine readable medium such as a storagemedium. Processors may perform the necessary tasks.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. For example, the above elements may merely be a component ofa larger system, wherein other rules may take precedence over orotherwise modify the application of the invention. Also, a number ofsteps may be required before the above elements are considered.Accordingly, the above description should not be taken as limiting thescope of the invention, which is defined in the following claims.

What is claimed is:
 1. A method for wireless communication over a satellite return link, the method comprising: receiving a signal comprising a linear combination of a plurality of orthogonal codes, the plurality of orthogonal codes transmitted by a plurality of calibration stations and received at a satellite by a plurality of satellite feeds, the satellite retransmitting the linear combination of the plurality of orthogonal codes, wherein at least two of the plurality of calibration stations are located within distinct satellite beam coverage areas; performing a correlation on the received plurality of orthogonal codes to obtain at least one of amplitude or phase information of signal paths originating from the plurality of calibration earth stations and passing through the satellite; estimating at least one of amplitude or phase effects of the satellite return link by applying an estimating function to the at least one of amplitude or phase information; and communicating on the satellite return link based at least in part on the at least one of amplitude or phase effects of the satellite return link.
 2. The method of claim 1, wherein the plurality of orthogonal codes are transmitted by the plurality of calibration stations at times when data is not being transmitted.
 3. The method of claim 1, further comprising: determining beamforming parameters for processing return link transmissions based at least in part on the at least one of amplitude or phase effects, the return link transmissions transmitted from user terminals in the plurality of satellite beam coverage areas and received at the satellite by a plurality of satellite feeds.
 4. The method of claim 3, further comprising: transmitting the beamforming parameters to the satellite.
 5. The method of claim 1, wherein either or both of the at least one of amplitude or phase effects are determined based on applying a weighted minimum mean square algorithm to either or both of the at least one of phase or amplitude information obtained from the received plurality of orthogonal codes.
 6. The method of claim 5, wherein weights for the weighted minimum mean square algorithm are determined based on a matrix of feed excitation levels.
 7. The method of claim 1, wherein the plurality of orthogonal codes comprise a plurality of pseudo-noise sequences.
 8. The method of claim 1, wherein each of the plurality of orthogonal codes are transmitted for a period of time less than 100 ms.
 9. An apparatus for wireless communication over a satellite return link, comprising: an antenna to receive a signal comprising a linear combination of a plurality of orthogonal codes, the plurality of orthogonal codes transmitted by a plurality of calibration stations and received at a satellite by a plurality of satellite feeds, wherein the satellite retransmits the linear combination of the plurality of orthogonal codes, and wherein at least two of the plurality of calibration stations are located within distinct satellite beam coverage areas; a correlator to correlate the received plurality of codes to obtain at least one of amplitude or phase information of signal paths originating from the plurality of calibration earth stations and passing through the satellite; a channel estimator to estimate at least one of amplitude or phase effects of the satellite return link by applying an estimating function to the at least one of amplitude or phase information; and a receiver to communicate on the satellite return link based at least in part on the at least one of amplitude or phase effects of the satellite return link.
 10. The apparatus of claim 9, wherein the plurality of orthogonal codes are transmitted by the plurality of calibration stations at times when data is not being transmitted.
 11. The apparatus of claim 9, wherein the channel estimator determines beamforming parameters for processing return link transmissions based at least in part on the at least one of amplitude or phase effects, the return link transmissions transmitted from user terminals in the plurality of satellite beam coverage areas and received at the satellite by a plurality of satellite feeds.
 12. The apparatus of claim 11, further comprising: a transmitter to transmit the beamforming parameters to the satellite.
 13. The apparatus of claim 11, wherein either or both of the at least one of amplitude or phase effects are determined based on applying a weighted minimum mean square algorithm to either or both of the at least one of phase or amplitude information obtained from the received plurality of orthogonal codes.
 14. The apparatus of claim 13, wherein weights for the weighted minimum mean square algorithm are determined based on a matrix of feed excitation levels.
 15. The apparatus of claim 9, wherein the plurality of orthogonal codes comprise a plurality of pseudo-noise sequences.
 16. The apparatus of claim 9, wherein each of the plurality of orthogonal codes are transmitted for a period of time less than 100 ms.
 17. An apparatus for wireless communication over a satellite return link, comprising: means for receiving a signal comprising a linear combination of a plurality of orthogonal codes, the plurality of orthogonal codes transmitted by a plurality of calibration stations and received at a satellite by a plurality of satellite feeds, the satellite retransmitting the linear combination of the plurality of orthogonal codes, wherein at least two of the plurality of calibration stations are located within distinct satellite beam coverage areas; means for performing a correlation on the received plurality of orthogonal codes to obtain at least one of amplitude or phase information of signal paths originating from the plurality of calibration earth stations and passing through the satellite; means for estimating at least one of amplitude or phase effects of the satellite return link by applying an estimating function to the at least one of amplitude or phase information; and means for communicating on the satellite return link based at least in part on the at least one of amplitude or phase effects of the satellite return link.
 18. The apparatus of claim 17, wherein the plurality of orthogonal codes are transmitted by the plurality of calibration stations at times when data is not being transmitted.
 19. The apparatus of claim 17, wherein the means for estimating determines beamforming parameters for processing return link transmissions based at least in part on the at least one of amplitude or phase effects, the return link transmissions transmitted from user terminals in the plurality of satellite beam coverage areas and received at the satellite by a plurality of satellite feeds.
 20. The apparatus of claim 19, further comprising: means for transmitting the beamforming parameters to the satellite.
 21. The apparatus of claim 17, wherein either or both of the at least one of amplitude or phase effects are determined based on applying a weighted minimum mean square algorithm to either or both of the at least one of phase or amplitude information obtained from the received plurality of orthogonal codes.
 22. The apparatus of claim 21, wherein weights for the weighted minimum mean square algorithm are determined based on a matrix of feed excitation levels.
 23. The apparatus of claim 17, wherein the plurality of orthogonal codes comprise a plurality of pseudo-noise sequences.
 24. The apparatus of claim 17, wherein each of the plurality of orthogonal codes are transmitted for a period of time less than 100 ms.
 25. A method for wireless communication over a forward satellite link, the method comprising: receiving signals comprising linear combinations of a plurality of orthogonal codes, the plurality of orthogonal codes transmitted by a plurality of gateways and received at a satellite by a plurality of satellite feeds, the satellite retransmitting the linear combinations of the plurality of orthogonal codes, the retransmitted linear combinations of the plurality of orthogonal codes being received by a plurality of calibration stations, wherein at least two of the plurality of calibration stations are located within distinct satellite beam coverage areas; performing a correlation on the received plurality of codes to obtain at least one of amplitude or phase information of signal paths originating from the plurality of gateways and passing through the plurality of feeds of the satellite; estimating at least one of amplitude or phase effects of the forward satellite link by applying an estimation function to the at least one of amplitude or phase information; and communicating on the forward satellite link based at least in part on the at least one of amplitude or phase effects of the forward satellite link.
 26. The method of claim 25, wherein the plurality of orthogonal codes are transmitted by the plurality of gateways at times when data is not being transmitted.
 27. The method of claim 25, further comprising: determining beamforming parameters for forward link transmissions based at least in part on the at least one of amplitude or phase effects, the forward link transmissions transmitted from the plurality of gateways to user terminals in the plurality of satellite beam coverage areas via the plurality of satellite feeds of the satellite.
 28. The method of claim 27, wherein the plurality of orthogonal codes are transmitted from the plurality of gateways without the beamforming parameters applied to the plurality of orthogonal codes.
 29. The method of claim 27, further comprising: transmitting the beamforming parameters to the satellite.
 30. The method of claim 25, further comprising: receiving, from the plurality of calibration stations, either or both of phase or amplitude shifts determined from correlations of the plurality of orthogonal codes received at the plurality of calibration stations in the linear combinations with the originally transmitted plurality of orthogonal codes.
 31. The method of claim 25, wherein either or both of the at least one of amplitude or phase effects are determined based on applying a weighted minimum mean square algorithm to either or both of the at least one of phase or amplitude information obtained from the received plurality of orthogonal codes.
 32. The method of claim 31, wherein weights for the weighted minimum mean square algorithm are determined based on a matrix of feed excitation levels.
 33. The method of claim 25, wherein the linear combinations of the plurality of orthogonal codes are generated at the satellite using a hybrid matrix.
 34. The method of claim 25, further comprising: sending the originally transmitted plurality of orthogonal codes to the plurality of calibration stations for use in the correlating to the plurality of orthogonal codes received in the linear combinations.
 35. An apparatus for wireless communication over a forward satellite link, comprising: an antenna to receive signals comprising linear combinations of a plurality of orthogonal codes, the plurality of orthogonal codes transmitted by a plurality of gateways and received at a satellite by a plurality of satellite feeds, the satellite retransmitting the linear combinations of the plurality of orthogonal codes, the retransmitted linear combinations of the plurality of orthogonal codes being received by a plurality of calibration stations, wherein at least two of the plurality of calibration stations are located within distinct satellite beam coverage areas; a correlator to correlate the received plurality of codes to obtain at least one of amplitude or phase information of signal paths originating from the plurality of gateways and passing through the plurality of feeds of the satellite; a channel estimator to estimate at least one of amplitude or phase effects of the forward satellite link by applying an estimation function to the at least one of amplitude or phase information; and a transmitter to communicate on the forward satellite link based at least in part on the at least one of amplitude or phase effects of the forward satellite link.
 36. The apparatus of claim 35, wherein the plurality of orthogonal codes are transmitted by the plurality of gateways at times when data is not being transmitted.
 37. The apparatus of claim 35, further comprising: determining beamforming parameters for forward link transmissions based at least in part on the at least one of amplitude or phase effects, the forward link transmissions transmitted from the plurality of gateways to user terminals in the plurality of satellite beam coverage areas via the plurality of satellite feeds of the satellite.
 38. The apparatus of claim 37, wherein the plurality of orthogonal codes are transmitted from the plurality of gateways without the beamforming parameters applied to the plurality of orthogonal codes.
 39. The apparatus of claim 37, wherein the transmitter transmits the beamforming parameters to the satellite.
 40. The apparatus of claim 35, wherein either or both of the at least one of amplitude or phase effects are determined based on applying a weighted minimum mean square algorithm to either or both of the at least one of phase or amplitude information obtained from the received plurality of orthogonal codes.
 41. The apparatus of claim 40, wherein weights for the weighted minimum mean square algorithm are determined based on a matrix of feed excitation levels.
 42. The apparatus of claim 35, wherein the linear combinations of the plurality of orthogonal codes are generated at the satellite using a hybrid matrix.
 43. A apparatus for wireless communication over a forward satellite link, comprising: means for receiving signals comprising linear combinations of a plurality of orthogonal codes, the plurality of orthogonal codes transmitted by a plurality of gateways and received at a satellite by a plurality of satellite feeds, the satellite retransmitting the linear combinations of the plurality of orthogonal codes, the retransmitted linear combinations of the plurality of orthogonal codes being received by a plurality of calibration stations, wherein at least two of the plurality of calibration stations are located within distinct satellite beam coverage areas; means for performing a correlation on the received plurality of codes to obtain at least one of amplitude or phase information of signal paths originating from the plurality of gateways and passing through the plurality of feeds of the satellite; means for estimating at least one of amplitude or phase effects of the forward satellite link by applying an estimation function to the at least one of amplitude or phase information; and means for communicating on the forward satellite link based at least in part on the at least one of amplitude or phase effects of the forward satellite link.
 44. The apparatus of claim 43, wherein the plurality of orthogonal codes are transmitted by the plurality of gateways at times when data is not being transmitted.
 45. The apparatus of claim 43, further comprising: determining beamforming parameters for forward link transmissions based at least in part on the at least one of amplitude or phase effects, the forward link transmissions transmitted from the plurality of gateways to user terminals in the plurality of satellite beam coverage areas via the plurality of satellite feeds of the satellite.
 46. The apparatus of claim 45, wherein the plurality of orthogonal codes are transmitted from the plurality of gateways without the beamforming parameters applied to the plurality of orthogonal codes.
 47. The apparatus of claim 46, wherein the means for receiving receives, from the plurality of calibration stations, either or both of phase or amplitude shifts determined from correlations of the plurality of orthogonal codes received at the plurality of calibration stations in the linear combinations with the originally transmitted plurality of orthogonal codes.
 48. The apparatus of claim 46, further comprising: transmitting the beamforming parameters to the satellite.
 49. The apparatus of claim 43, wherein either or both of the at least one of amplitude or phase effects are determined based on applying a weighted minimum mean square algorithm to either or both of the at least one of phase or amplitude information obtained from the received plurality of orthogonal codes.
 50. The apparatus of claim 49, wherein weights for the weighted minimum mean square algorithm are determined based on a matrix of feed excitation levels.
 51. The apparatus of claim 43, wherein the linear combinations of the plurality of orthogonal codes are generated at the satellite using a hybrid matrix. 